For ideal stereo listening, the two stereo channels should be identical electrically and acoustically, and the stereo loudspeakers should be optimally and symmetrically located in a symmetrical listening room where the listener is located centrally between the two loudspeakers at an optimal distance from the loudspeakers. Under these ideal conditions a listener experiences accurate image localization, i.e. the ability to sense good approximations of each sound source location as originally recorded, perceive the on-axis frequency response of the loudspeakers, and additionally, experience the sensation of sound ambience or spaciousness of the recording environment which is generally much larger than the listening room.
Imaging is a function of the relative amplitudes and phase of the right and left acoustic signals as perceived at each ear. Additionally, the aural mechanism of imaging is frequency dependent, acting predominantly within a mid range of the audio spectrum, e.g. 300 to 1,000 Hz, where the wavelength is equal to or greater than the distance between the listener's ears. At higher frequencies, the short wavelengths can produce confusing multiple inter-aural polarity inversions and therefore only interaural amplitude differences are perceived by the hearing mechanism and contribute to the imaging effect at such frequencies.
There are many situations where it is impossible to realize ideal listening conditions, for example in an automobile where the space is highly restricted; and, even though the loudspeakers can be located symmetrically within the available space, proper imaging is generally possible only at the centerline of the vehicle. Further, in bucket seat automotive arrangements, the centerline listening position is not accessible and both the driver and the passengers suffer the compromise of an unbalanced listening location where the imaging perception is substantially degraded. Adjusting the stereo amplitude balance control of a conventional auto stereo system to favor one of the front seat locations, e.g. the driver's location, can provide some improvement for the driver by balancing the left and right channel amplitudes as perceived, but fails to provide optimal image perception. This failure is due to the (a) difference in L and R sound travel path lengths and resulting polarity inversions in the critical 300 to 1,000 Hz region, (b) the severely unbalanced off-axis listening angles relative to each loudspeaker resulting in an unbalance of the perceived high frequency levels from the loudspeakers and (c) the greater degree of ambient sound, i.e. reverberant sound fields, produced by the further loudspeaker relative to the closer loudspeaker at the described asymmetrical listening locations.
It is known that cross-mixing the two stereo channels together in additive polarity will reduce the stereo separation effect perceived by a listener: carried to the limit, full L+R addition in both channels reduces stereophonic sound to monophonic. It is also known that cross-mixing in subtractive polarity, by decreasing the common-mode signal content in each channel, can create a perception of "musical stage expansion" and enhance ambient sound reproduction.
It is known that, for particular non-ideal listening locations, subjective improvements in the imaging and/or ambience of sound reproduction may be realized through signal processing of one or both of the stereo channels. Stereo modification systems have been proposed and utilized which alter the right and left stereo source signals in various ways; however such systems fail to compensate for each of the previously described deficiencies and signal errors which occur under non-ideal listening conditions, and, in cases where digital processing is required, also tend to be substantially more complex and costly to implement relative to the present invention.